GB2183417A - Laser rangefinder - Google Patents

Abstract

A laser rangefinder comprises a semiconductor laser diode (2), a transmission/receiver lens (1), an integrated optics waveguide switch (3) and a photodetector (4). In the transmit mode the laser and one arm (11) of the waveguide switch (3) and an intermediate single mode optical fibre (8) are coupled to form an extended-cavity laser oscillator. In the receive mode the laser acts as an optical amplifier for received pulses and is coupled to the photodetector. Switching of the waveguide switch, by application of a pulse to the gating input, causes switching between the receive and transmit modes. The semiconductor laser diode is permanently current biassed above its oscillation threshold. <IMAGE>

Description

SPECIFICATION Rangefinder This invention relates to rangefinders and in particular to semiconductor laser rangefinders.

According to the present invention there is provided a laser rangefinder including a semiconductor laser, a waveguide switch and a photodetector, wherein in a transmit mode the laser and the waveguide switch are coupled to form an extended-cavity laser oscillatorto provide optical transmission pulses and wherein in a receive mode the laser acts as an optical amplifierfor received pulses priorto application thereoftothe photodetector, switching of the waveguide switch in use ofthe rangefinder serving to cause switching between the transmit and receive modes.

Embodiments ofthe invention wiil now be described with reference to the accompanying drawings, in which: Figure 1 shows schematically a first embodiment of semiconductor laser rangefinder, and Figure2 shows schematically a second embodiment of semiconductor laser rangefinder.

The semiconductor laser rangefinder illustrated in Figure 1 comprises a transm itting/receiving lens 1, semiconductor laser diode 2, which mayforexample be of GaAIAs of InGaAsP, an integrated optics waveguide switch (coupler) 3 and a photodetector 4.

A monomode optical fibre 5 is coupled between one end 6 of one waveguide arm 7 of switch 3 and the photodetector4. One end of a monomode optical fibre 8 is coupled to the other end 9 ofthe one waveguide arm 7. The other end 10 ofthe optical fibre 8 is coupled to the laser diode in a low loss manner and may be disposed closely adjacent one output facet of the laser diode 2 and lensed.

Alternatively fibre end 10 may be non-lensed.Any gap between the fibre and laserdiode2maybefilled with an index matching medium. The fibres 5 and 8 may be coupled to the switch 3 in a permanent low loss manner by, for example, a suitable cement. The other waveguide arm 11 has a reflective coating 12, for example, of gold at one end, that is itis terminated by the reflective coating. The one output facet of the laser diode 2 adjacent fibre 8 is provided with an anti-reflection coating, for example of silicon nitride or aluminium oxide. The waveguide switch 3 is provided with electrode means 14forthe application of a gating input.Typically the waveguide switch is of Ti: LiNbO3, that is composed of LiNbO3with titanium diffused waveguide regions.

The arrangement of the elements ofthe rangefinder device of Figure 1 is such that the laser diode operates alternately to generate optical pulses for transmission towards atarget 13 and as an optical amplifierfor a returned signal. The laser diode 2 is DC biassed above its threshold for oscillation in the extended cavity formed by the laser diode itself, the single mode coupling fibre 8 and the waveguide arm 11 having the reflective coating, which arm is switched in by the appropriate electrical gating pulse to the gating inputofthe switch 3forthe duration of that pulse, when the laser acts as an extended cavity pulsed laser oscillator and generates optical transmission pulses.In the absence of such an electrical gating pulse the laser diode is optically coupled to the photodetector and provides an optical gain, determined by the DC bias current, to any reflected signal, that is it acts as a single-pass optical gain element, amplifying received pulses reflected from the target.

Thus the laser diode is switched between receiving and transmitting modes by the application of a switching voltage to the integrated optics switch 2.

Short electrical switching pulses applied to the switch can be used to generate short duration Q-switched optical pulses of high peak power.

Alternatively, the integrated optics switch 2 can be switched by a sinusoidal RF cu rrent at a frequ ency given bythe inverse round-triptime in the extended optical cavity. In this case a burst of ultra-short mode-locked optical pulses is generated and transmitted to thetarget.

In operation a transmission pulse, or pulse burst, is followed buy a period when the rangefinderis switched to the receiving mode and the photodetector output corresponds to received reflected signals (pulses). The duration ofthe receiving mode, that is the reception period, depends on the maximum detectable target range.

Post-detection timing electronics (not shown) may beusedtocomputethetargetrangewith a resolution which is inversely dependent on the length of the transmitted optical pulse or pulse burst.

Whereas in the embodiment of Figure 1 the straight-through or normally cou pled arm 7 ofthe integrated optics switch is coupled to the fibre coupled photodetector, alternatively the reflective coating may be disposed at end 6 of arm 7 and the fibre-coupled photodetector coupled to end 12 of waveguide arm 11, in which case in the absence of an electrical gating pulse the laser is operated as the extended-cavity pulsed laseroscillatorthat is itis in itstransmission mode,and application ofthegating pulse to switch 3 switches the laserto its receiving mode.

Conventional laser rangefinders usually require either a separate receiving system, or, if a common transmitting/receiving aperture is used, some form of isolation, for example polarisation discrimination, between transmitted and received pulses. The arrangement provided by the present invention involves the use of a common aperture and a common optical gain-element system which provides automatic isolation ofthe receiving photodetector, whilst short, high peak power optical pulses are generated fortransmission, The isolation ofthe photodetector during transmission allowsthe laser diode to be permanently current-biassed above its oscillation threshold, increasing both the transmitted pulse power and the optical gain experienced by received signals.This is in constrast to a conventional approach in which the laser is biassed below threshold and electrically gain switched into oscillation, causing saturation ofthe receiver electronics and reducing the available optical signal gain.

An improvement in detected signal-to-noise ratio may be provided using the basic approach ofthe present invention. Instead of a single laser diode, which has a limited output, multiple 0switch semiconductor lasers may be employed. These may transmit either through separate transmitter/receiver lenses or, and as illustrated in Figure 2,through a single fibre-coupled lens 20. In Figure 2there is illustrated a plurality (n) of laser diodes 21 coupled together by an n-waymultimode fibre coupler 22. Associated with each laser diode 21 is a respective integrated optic switch/coupler 24 which may beformed as an arrayofcouplers( coupler) on a common substrate 25 as illustrated.

The gating inputs to the couplers 24 are coupled to a common gating input. The laser diodes 21 are coupled to the couplers 24 by respective monomode fibre links 26. Similarlyto Figure 1,thethrough arms 23 ofthe couplers 24 are coupled via anothern-way multimode fibre coupler 29 to a photodetector 27 which is common to all couplers 24. The dtherarms ofthe couplers each have a reflective coating 28. As described with reference to Figure 1,alternatively thethrough arms may have the reflective coatings and the other arms be connected to the photodetector. In the arrangement of Figure 2 each extended-cavity pulse laser oscillator generates respective optical transmission pulses so that the transmitted power is increased in comparison with that provided bythe Figure 1 arrangement.Received pulses are amplified in parallel in the multiple laser gain elements before being applied to the single photodetector 27. If mode-locked operation is used that is the couplers are switched by sinusoidal RF current, as above, the time-i nteg ration ofthe received pulse-burst may allow an additional improvement in signal detectability.

In principle laser diode wavelengths of 0.8, 1.3 and 1 .5im are all usable, although because ofthe susceptability of LiNbO3 to optical damage at short wavelengths and the availability of suitable lasers at this time, in practice choice may be restricted to 1.3,u m.

The use ofthe laser diode in the receiving mode as a single pass optical gain element means that amplification of 100 to 1000 or so can be achieved which increases the maximum range oftargets detectable bythe rangefinderfrom say 500 meters to 3km. The overall rangefinder may be made by lightweight and rugged, making it particularly suitable for portable rangefinder applications, for example. In the case of "non-cooperative" targets, for example militarytargets,the maximum range may be only of the order of 500 meters, however, there is then a high range resolution of a few cm, for example.

Claims (13)

1. A laser rangefinder including a semiconductor laser, a waveguide switch and a photodetector, wherein in a transmit mode the laser and the waveguide switch are coupled to form an extended-cavity laseroscillatorto provide optical transmission pulses and wherein in a receive mode the laser acts as an optical amplifierfor received pulses prior to application thereofto the photodetector, switching ofthe waveguide switch in use of the rangefinderserving to cause switching between the transmit and receive modes.

2. A laser rangefinder as claimed in claim 1 wherein the semiconductor laser comprises a laser diode permanently current biassed, in use ofthe rangefinder, above its oscillation threshold.

3. A laser rangefinderas claimed in claim 1 or claim 2, wherein switching of the waveguide switch is effected by the application of short electrical switching pulses to a gating inputthereofwherebyto provide short duration Switched optical transmission pulses.

4. A laser rangefinder as claimed in claim 1 or claim 2, wherein switching of the waveguide switch is effected by the application of a sinusoidal RF currentto a gating inputthereofwherebyto provide a burst of ultra-short mode-locked optical transmission pulses.

5. A laser rangefinder as claimed in any one of the preceding claims wherein the waveguide switch includes two waveguide arms one of which is coupled to the photodetector and the other of which is terminated by a reflective coating, and wherein in the transmit modethe semiconductor laser is coupled to the other waveguide arm and in the receive mode the semiconductor laser is coupled to the one waveguide arm.

6. A laser rangefinder as claimed in claim 5 wherein the semiconductor laser is coupled to one of said one or otherwaveguide arms of the waveguide switch by a length of monomode optical fibre.

7. A laser rangefinder as claimed in any one of the preceding claims and including a lens via which optical pulses are transmitted to a target from the semiconductor laser and via which received optical pulses reflected from the target are applied to the semiconductor laser.

8. A laser ra ngefinder as claimed in a ny one of claims 1 to 6, including a plurality of parallel arrangements of a said semiconductor laser waveguide switch, which parallel arrangements are coupled to a common photodetector comprising said photodetector.

9. A laser rangefinder as claimed in claim 8 wherein each parallel arrangement includes a respective lens via which optical pulses are transmitted to a target from the respective semiconductor laser and via which received optical pulses reflected from the target are applied to the respective semiconductor laser.

10. A laser rangefinder as claimed in claim 8 wherein a common lens is provided for the transmission of optical pulses from the plurality of semiconductor lasers to a target and forthe reception of optical pulses reflected from the target, the semiconductor lasers being coupled to the common lens via a multiwayopticalfibrecoupler.

11. A laser rangefinder as claimed in any one of the preceding claims wherein the photodetector is coupled to the or each waveguide switch by optical fibre.

12. Alaserrangefinderasclaimed in any one of the preceding claims wherein the or each semiconductor laser is a GaAIAs or InGaAsP laser diode and wherein the or each waveguide switch is comprised of LiNbO3 with Ti diffused waveguides.

13. A laser rangefindersubstantially as herein described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.